1. Field of the Invention
The present invention relates to an autofocus imaging optical system which includes a splitting optical system in an optical path, splits a light beam in an image pickup optical system for focus state detection and detects a focus state of an image pickup lens, and to an optical apparatus, image pickup device and the like including the autofocus imaging optical system.
2. Description of the Related Art
For auto-focusing (AF) technologies in a photographic device such as a still camera or video camera, various approaches have been proposed hitherto. Especially, for example in Japanese Patent Application Laid-Open Nos. 2003-279842, 2003-279846, 2003-279847 and 2003-287673, a lens-image pickup device which includes a splitting optical system in a optical path and AF detection means provided in a split optical path is disclosed.
However, if a light beam used for image pickup is split for focus detection as described above, transmittance in an image pickup device may decrease and T number may increase. Therefore, an image becomes darker or an exposure time increases due to the insufficient light amount. Then, a more amount of light for focus detection may be required to improve focus accuracy or to enable to range the subject in a low illuminance condition. Accordingly, the image may further become darker.